Size, position and window maximize state are saved in gsettings
when closing Fractal. They are then restored on startup.
Window position is not working on all windowing systems.
Fixes#123
Adds a gschema (org.gnome.Fractal.markdown-active) to remember the state
of the markdown toggle. Every time the app starts up it checks the
value of the schema. Every time the switch is toggled the schema's
value changes.
Related to #229
